How to lower VPD?

[Ca++]

There seems to be some confusion. Some folks are recommending humidification, and others de-humidification.

He wants to lower VPD, which is a measure of the difference between 100% RH and actual RH. His difference is too great. His RH just 30%

I really don't like VPD conversations in a home grow (or scaled up home grow) situation. It's a bit too scientific, where there is no strong science to follow. It's just calculated from what we do actually measure. Temperature and humidity. I'm quite happy measuring the two separately, and thinking of them holistically, without actually going to the trouble of making math out of the situation. I believe this is the reality in most of our grows, and VPD is just an interesting science paper.

The measurement of concern is the 30% RH. It's low because of the extraction rate to keep the temp down. I would lower the extraction rate, because raising the RH is more important than keeping the temp where it is. As the temp in the tent gets further above room temp, the heat loss through the walls will increase. Baking a tent is actually hard work. Further to this, a fan blowing on the outside of the tent will ensure the greatest temp difference possible. The airflow up through the flaps and out the top is often messed with by circulation fans. The lights heat wants to be up and out. Realistically these plants need to keep their moisture around them, so you could turn them off. Another idea would be turning the light down in order to get the RH where the plants can grow. At 30% RH there is no need for huge amounts of light. The plants are not breathing co2 properly.

Literally sacrifice everything you have to, to get the RH right. It's of prime importance.

 

[Switcher56]

Seems to me he is controlling the whole chibang with an "oversized fan" controller. A dehuey not req'd at this point, since his fan seems to control that. But he definitely needs an RH controller and a humidifier. Control the humidity with the latter and tweak with the fan speed. The 8" fan displaces a lot of volume, might just need to be set at lowest setting when running. Simply put, you can't ask one tool to control everything.

 

[flylowgethigh]

I use a window banger set at 67* and a dehuey which likes to heat air coming out to 78*. They balance the lung room air, which I force into the tent, and the tent is 75*. When the lights go out I set the window AC to 75*. That keeps the VPD around 1.30-1.40, mid to late flower. I would like less humidity. The tent blows to outside, and the lung room make-up air coming in from the halllway is too humid. That will take a house dehuey.

Controlling the humidity is really important. Having low ambient humidity makes the job a lot easier. I live in a steamroom called the Southeast. Humidity is king here, high temps queen.

 

[Ca++]

I'm not sure I could work with a fan controller that has both RH and temp settings. There is a priority issue.

 

[Switcher56]

Yup! (marketing)

 

[Ca++]

As you and others have outlined, a humidifier sorting out the air that's about to get drawn in, is the right idea. That humidifier in the room needs a RH stat. Then as the air speed through the tent changes, the humidifier will produce the amount of moisture required to enrich that air volume. In practice, cooler nights will see the extractor slow and the humidifier run less often. Then lights on, the extractor will speed up, and the humidifier may have to run continuously. That is about the ideal situation. You set the actual vapour stream size on the hummer so that it runs continuously at your max extraction speed. Using the RH stat to turn it off, only when it's too damp.

It's hard to see, but air drawn into the room from the corridor is a different temperature to that in the room. It needs mixing with fans. If you don't mix it with room air, it will slip along the floor and into the tent, as if it were ducted. That means we must mix it with the room air ourselves. Then the tent gets our treated air. The bigger the room, the less problem a cycling hummer causes. However, setting the stream size to the minimum required to work, is generally ample. Giving a nice continuous treatment.


Temperature monitoring fan speed controllers are great in a greenhouse where cloud cover can see temps swings of a large proportion. Indoors, the average home grower has quite consistent weather conditions. I have spent perhaps $500 in such temperature monitoring controllers. Then over time, swapped back to manual controllers. Though I have a number of RH stats I couldn't be without. Just sat on hummers, treating inlet air. Which is steady state operating that any numpty can tweak to get right. Even me

 

[Three Berries]

I use both the RH and Temp settings. There also are temp and humidity point differentials offsets. Basically it lets the fan run at lower speeds but does not ever really hit the target setting. Works good when the RH is hard to keep up and the temp is the trigger. Allows the fan to run constant at a slower speed yet still controlled and the speed can be raised or lowered. Conserves the humidity and doesn't do complete flushes like on and off settings do with temp.

When ambient humidity is high and constant fan running won't drop it, I then run more lighting as a heater. Then play with the settings for control.

 

[Rtaym22224]

I use both the RH and Temp settings. There also are temp and humidity point differentials offsets. Basically it lets the fan run at lower speeds but does not ever really hit the target setting. Works good when the RH is hard to keep up and the temp is the trigger. Allows the fan to run constant at a slower speed yet still controlled and the speed can be raised or lowered. Conserves the humidity and doesn't do complete flushes like on and off settings do with temp.

When ambient humidity is high and constant fan running won't drop it, I then run more lighting as a heater. Then play with the settings for control.

I have two sensors connected to my grow light master controller that are picking up temps and humidity at canopy level.
These sensors show humidity of 53% at the moment and a temp of 74.9.

What is the difference between relative and ambient humidity?

My fan controller has a vpd temp offset “adjusts the leaf temp in vpd calculation”, how do I use this or do I even need to?

 

[flylowgethigh]

I have two sensors connected to my grow light master controller that are picking up temps and humidity at canopy level.
These sensors show humidity of 53% at the moment and a temp of 74.9.

What is the difference between relative and ambient humidity?

My fan controller has a vpd temp offset “adjusts the leaf temp in vpd calculation”, how do I use this or do I even need to?

Are we supposed to be using leaf or air temp to figure VPD?

I have the leafs at 72 while the air is 75

 

[flylowgethigh]

Are we supposed to be using leaf or air temp to figure VPD?

I have the leafs at 72 while the air is 75. Humidity is what I need more control of, and that means LESS moisture. Hard in the summertime with outside ambient rh around 100%

 

[Three Berries]

Unless you can monitor the leaf temp constantly like air temp, then it's just guessing if trying to control with that input.

But They say to use leaf temp.

 

[flylowgethigh]

If it's leaf temp we want I am 3* too cool. Maybe I need to warm the tent up a notch.